There are many applications for vaporizers of various sorts. In the broadest sense, a vaporizer converts a liquid to a vapor or a two phase mixture of liquid and vapor, or may convert a two phase mixture of vapor and liquid to a wholly, single phase vapor. In some instances, vaporizers may also provide a conversion from the solid phase to the liquid phase en route to providing a vapor or a two phase liquid/vapor mixture.
In some vaporizers, the material to be vaporized is brought into contact with a hot surface that may be heated by a nonfluid medium as, for example, an electrical heating element or by radiation impinging upon the heating surface. In most other cases, however, the vaporization occurs by placing the material to be vaporized on one side of a thermally conductive separator and locating a hot heat exchange fluid on the other side of the separator plate. The heat exchange fluid may be heated gases produced by a chemical reaction or simply some liquid or gaseous fluid that had previously been heated by a reaction or even a heating element or the like. In some cases, the heat exchange fluid may be relatively stationary. However, in many cases, it is desired that the heat exchange fluid be moving to induce turbulence and improve heat exchange coefficients. Generally speaking, it is desirable that the heat exchange fluid be moving countercurrent to the liquid to be vaporized for maximum heat transfer efficiency.
One application for a vaporizer of the latter type is in a particular type of fuel cell system. As is well known, fuel cell systems are attracting considerable attention as an efficient and nonpolluting means of providing traction power for vehicles. Some fuel cell systems operate using relatively pure hydrogen as a fuel source while others utilize a hydrogen rich stream of fuel. Many of the latter type of fuel cell systems are so-called reformer type fuel systems which is to say that they are provided with a hydrogen rich fuel which is then reformed into an even richer hydrogen stream which in turn is passed to the fuel cell of the system. Fuels include methanol, ethanol, gasoline and the like.
In such systems, the fuel is a hydrogen containing liquid which must be vaporized before it is passed on to the system reformer to be reformed into a hydrogen rich gas. One example of a fuel vaporizer that is ideally suited for use in reformer type fuel cell systems is disclosed in the commonly assigned application of Michael J. Reinke et al, Ser. No. 10/145,531, filed May 14, 2002, entitled “Method and Apparatus for Vaporizing Fuel for a Reformer Fuel Cell System” (Attorney's Docket No. 655.00937), the entire disclosure of which is herein incorporated by reference.
It has been found that when a fuel cell system of the reformer type is to be employed in a vehicular application, the fuel charge in the vaporizer should be as small as possible to minimize the time required for the system to respond to a change in load. As is well known, drivers of vehicles propelled by internal combustion engines expect immediate response when they step on the gas pedal or a fuel feed to accelerate as when passing another vehicle. In vehicles propelled by fuel cell systems, a similar response is expected when the driver steps on the fuel cell system equivalent of the gas pedal. As it happens, the greater the charge of fuel in the fuel vaporizer, the slower the response of the fuel cell system to demands of the operator. At the same time, the fuel must be completely vaporized prior to being passed to the system reformer. One way of increasing the effectiveness of the vaporizer is to increase the core length, that is, the length of that part of the heat exchanger housing the fuel flow path and the heated fluid medium path which are in heat exchange relation. However, as this length is increased, response time also increases as a result because the fuel charge in the vaporizer increases as a result of the increased volume that accompanies increased length.
Consequently, to achieve improved efficiency in vaporizers intended for use in such systems, relatively high temperature differentials may be employed to increase the rate of heat transfer within the vaporizer and thus increase the rate of vaporization of the liquid fuel. This, in turn, increases the susceptibility of the vaporizer to thermal fatigue. Since thermal fatigue reduces the useful life of the vaporizer, it is desired to eliminate or minimize thermal fatigue without increasing the system response time, particularly when the fuel cell is employed in a vehicular application.
The present invention is directed to achieving that goal.